Fish in general, but especially freakish oversized fish, aren’t exactly designed to support their own weight out of the water. In the water, their density gives them near neutral bouyancy, but out of it their own weight probably causes quite a bit of damage. Same reason that 20 pound lobster probably died, and the same reason our own brains are housed in a liquid filled cranium.
It would have been pretty amazing had the fish lived, taking this into consideration. I know other fish this big have lived (the Texacoma fish) and that’s actually the more remarkable story.
I’m more excited by the fish’s existence in the first place than saddened by its death – and happy that people got a chance to see it at all. Now let’s see a flathead that size!